Glucoamylases (1,4-.alpha.-D-glucan glucohydrolase, EC 3.2.1.3) are enzymes which catalyze the release of D-glucose from the non-reducing ends of starch or related oligo- and polysaccharide molecules.
Glucoamylases are produced by several filamentous fungi and yeasts, including Aspergillus niger and Aspergillus awamori.
Commercially, the glucoamylases are used to convert corn starch which is already partially hydrolyzed by an .alpha.-amylase to glucose. The glucose may further be converted by glucose isomerase to a mixture composed almost equally of glucose and fructose. This mixture, or the mixture further enriched with fructose, is the commonly used high fructose corn syrup commercialized throughout the world. This syrup is the world's largest tonnage product produced by an enzymatic process. The three enzymes involved in the conversion of starch to fructose are among the most important industrial enzymes produced.
One of the main problems existing with regard to the commercial use of glucoamylase in the production of high fructose corn syrup is the relatively low thermal stability of glucoamylases, such as the commercially available Aspergillus niger glucoamylase (i.e., (sold as AMG by Novo Nordisk A/S). The commercial Aspergillus glucoamylase is not as thermally stable as .alpha.-amylase or glucose isomerase and it is most active and stable at lower pH's than either .alpha.-amylase or glucose isomerase. Accordingly, it must be used in a separate vessel at a lower temperature and pH.
U.S. Pat. No. 4,247,637 describes a thermostable glucoamylase having a molecular weight of about 31,000 Da derived from Talaromyces duponti suitable for saccharifying a liquefied starch solution to a syrup. The glucoamylase is stated to retain at least about 90% of its initial glucoamylase activity when held at 70.degree. C. for 10 minutes at pH 4.5.
U.S. Pat. No. 4,587,215 discloses a thermostable amyloglucosidase derived from the species Talaromyces thermophilus with a molecular weight of about 45,000 Da. The disclosed amyloglucosidase (or glucoamylase) loses its enzymatic activity in two distinct phases, an initial period of rapid decay followed by a period of slow decay. At 70.degree. C. (pH=5.0) the half-life for the fast decay is about 18 minutes with no measurable loss of activity within an hour in the second phase of decay. Bunni L et al., (1989), Enzyme Microb. Technol., Vol. 11, p. 370-375. concerns production, isolation and partial characterization of an extracellular amylolytic system composed of at least one form of .alpha.-amylase and one form of an .alpha.-glucosidase produced by Talaromyces emersonii CBS 814.70. Only the .alpha.-amylase is isolated, purified and characterized.